In a number of modern accelerator based applications, there is growing need for photocathode electron sources that can achieve high brightness and high-average-current, while at the same time lasting for long lifetimes. Examples of such applications include, without limitation, high-average-power free electron lasers (FELs), electron ion colliders (EICs), coherent electron coolers (CECs) and other energy recovery linac (ERL) based applications.
Typically, photocathodes can be used in devices such as RF guns and DC voltage guns. To achieve a high-brightness beam, the laser spot size (i.e. cathode emission spot) should be small, so that the thermal emittance is low. To achieve high-average-current, a high bunch charge and high repetition rate is needed. A high bunch charge beam generated with a small laser spot size requires a high field gradient on the cathode, in order to reduce space charge effects near the cathode. A high field gradient, however, can cause electrical breakdown issues. Another significant problem with semiconductor photocathodes is their short lifetime, due to their high sensitivity to the vacuum. These cathodes cannot operate with very high average current.
A number of approaches have been developed to overcome the insufficient charge lifetime (QT) problem of photocathodes. These approaches suffer, however, from a number of problems, including without limitation: increases in emittance; cathode center damage due to ion back-bombardment; and laser heating from very high power lasers, which may overheat or even crack the cathode.